This invention relates to methods and systems for molding thermoplastic sandwich material and deep-drawn articles produced by such methods and systems.
Thermoplastic Sandwich Materials
The stiffness of thermoplastic sandwich materials is due to the distance between its two skins that gives a high moment of inertia to the part. It is therefore important to preserve the distance between the skins during the molding of the sandwich part in order to obtain the full stiffness potential of the part. Any reduction of thickness causes a reduction of stiffness.
During the conception of a sandwich part with a cellular core, it is sometimes necessary to reduce the thickness of the part locally in order to obtain a specific shape. The stiffness in that area will be reduced but can be maintained by the part geometry. An example is that of a suitcase corner that gives the stiffness to the suitcase even though the part thickness is uniform.
Thermoplastic sandwich materials are traditionally used as flat panels or shaped parts for the following applications:
Currently, the market for sandwich materials is:
Competitive cost for flat panels,
Very high cost and low volume for shaped sandwich parts limits market to high technology applications and not high volume parts such as automotive parts.
Deep-draw Stamping of Steel Sheets
A stamped steel part is deep drawn wherein the depth of the part is typically more than one hundred times its thickness. Deep-draw of steel is possible because of its ductility.
Technical solutions were developed such as the use in the mold of a blank holder at the periphery of the stamping area that allows the part to be deformed under a high pressure without any folds. Folding the excess material of the sheet rather than stretching it is avoided because the created folds are brittle in fatigue and aging.
The pressure of the blank holder is a function of:
Initial blank diameter and its thickness after stamping;
Tensile strength of the steel;
Diameter of the stamping mold; and
An empirical parameter, n, that is a function of the ratio of blank diameter to stamping die diameter.
Stamping of a Cylindrical Steel Part
To manufacture cylindrical parts starting from a steel blank, it is possible to obtain a part with a maximum depth that is equal to seven times its diameter but six successive stampings are needed.
As an example, to obtain a cylindrical part with a diameter of 100 mm and a depth of 700 mm, eight stages are required as follows:
1. Cut a circular blank of 538 mm diameter;
2. Stamp to obtain a part with a 325 mm diameter and a depth of 145 mm;
3. Stamp to obtain a part with a 240 mm diameter and a depth of 240 mm;
4. Stamp to obtain a part with a 194 mm diameter and a depth of 325 mm;
5. Stamp to obtain a part with a 155 mm diameter and a depth of 428 mm;
6. Heat treat the part;
7. Stamp to obtain a part with a 124 mm diameter and a depth of 553 mm; and
8. Stamp to obtain a part with a 100 mm diameter and a depth of 700 mm.
After the fifth stage it is necessary to heat treat the part to regenerate the metal.
Stamping of a Part With Four Corners
The technique is similar to that used for cylinders except for the shape of the initial blank to avoid too much material loss after stamping. To define the development of the corners and of the walls, curves based on experience exist to define the depth and the radius.
Stamping of Sandwich Steel Blanks
Sandwich steel blanks are made of a sandwich with thin skin layers with a core. The blanks can be stamped.
Processing of Flat Thermoplastic Sandwich Materials
European Patent EP 0 649 736 B1 explains the principle of the sandwich technique for forming substantially flat parts. The part is made in a single stage by pressing in a cold mold, at a pressure in the range of 10 bars to 30 bars, a stack consisting of at least a first outer skin layer of stampable reinforced thermoplastics material, a cellular core of thermoplastics material, a second outer skin layer of stampable reinforced thermoplastics material. The axes of the cells of the cellular core are generally oriented perpendicular to the skin layer. The skin layers and core are previously heated outside the mold to a softening temperature.
European patent application 894611 A1 describes a 2D deep-draw which utilizes an element such as a piece of EPP foam to make sure that a skin is not in contact with a core during heating. When the blank is stamped, the tension on the skin is such that it does not crush the core during forming.
Shaping of Sandwich Material by 3D Deep-draw Solutions Used for Sandwich Material in the Aeronautics Industry
These solutions consist in erasing the problems due to:
The high stiffness of the skins in elongation,
Low compression strength of the core as compared to the tensile strength.
Therefore, to form a corner in a sandwich part, Hexcel Composites Company cuts the skin, and adds epoxy adhesive to fill the honeycomb.
If the corner will be exposed to high stress, the solution in the aeronautics industry is:
either the corner is reinforced with an angled insert(s) that is glued, or
the corner is reinforced with a metallic insert (extruded) with which two sandwich panels are assembled (glued).
Deep-drawing of Fabric-Reinforced Thermoplastics
Some experimental work has been done at the German Institute IVW (Kaiserslautern Germany) on the deep-drawing of fabric reinforced thermoplastics. A summary of that work is presented in the journal POLYMER COMPOSITES, August 1996, Vol. 17, No. 4, p. 643-647 by Breuer, Neitzel, Ketzer and Reinicke. They explain that for stamp forming of fabric-reinforced sheet material into simple 3D parts ordinary planar blank holders have proved to be sufficient to avoid any wrinkling. However, to stamp more complex shaped and large parts, this technique is limited. In this case, different clamping forces are needed on different areas of the blank. They propose a flexible roller-tracking device where the sheet material is drawn into the mold between two knurled rollers. With this system, different tension forces can be applied at selected zones of the sheet.
An object of the present invention is to provide a method and system for molding thermoplastic sandwich material and a deep-drawn article produced thereby wherein the thickness of at least one side wall of the deep-drawn article is substantially the same as the original thickness of a blank of the thermoplastic sandwich material.
Another object of the present invention is to provide a method and system for molding thermoplastic sandwich material and a deep-drawn article produced thereby wherein the articles are produced at a relatively low cost and high volume (i.e. low cycle time) by utilizing a single stamping step.
In carrying out the above objects and other objects of the present invention, a method is provided for molding thermoplastic sandwich material to form a deep-drawn article. The method includes positioning a blank of thermoplastic sandwich material having a cellular core over a female die having an article-defining cavity defined by inner surfaces of the female die. The method also includes forcing an inner portion of the blank into the female die along a substantially vertical axis and against the inner surfaces of the female die to obtain deep-drawn material. The method further includes clamping at least one outer portion of the blank immediately adjacent the female die to guide the at least one outer portion of the blank to travel into the article-defining cavity at an acute angle with respect to the vertical axis during the step of forcing so that thickness of at least one side wall of the deep-drawn material is substantially the same as thickness of the blank of thermoplastic sandwich material and so that the deep-drawn material does not significantly stretch or tear during the step of forcing. The method still further includes removing the deep-drawn material from the female die, and removing any excess material from the periphery of the deep-drawn material to form the deep-drawn article.
Preferably, the step of forcing includes the step of stamping and the step of clamping is performed with a clamping force which increases during the step of forcing.
The step of clamping may be performed at a plurality of spaced outer portions of the blank immediately adjacent the female die. Preferably, the step of clamping is performed at the plurality of spaced outer portions by a clamp assembly and a counter clamp assembly and wherein the method further includes maintaining a substantially constant distance between the assemblies during the step of forcing.
Preferably, each of the assemblies includes a plurality of spaced elongated clamping surfaces for clamping the plurality of spaced outer portions of the blank wherein each elongated clamping surface is inclined at the acute angle with respect to the vertical axis.
Preferably, the step of forcing is performed in a single stamping stage.
The inner surfaces of the female die may define a plurality of corners which correspond to spaces between the plurality of spaced outer portions wherein the spaces are sized to permit thermoplastic sandwich material of the blank to move therein during the step of forcing.
Further in carrying out the above objects and other objects of the present invention, a system is provided for molding thermoplastic sandwich material to form a deep-drawn article. The system includes a female die having an article-defining cavity defined by inner surfaces of the female die and a male die for forcing an inner portion of a blank of thermoplastic sandwich material having a cellular core into the female die along a substantially vertical axis and against the inner surfaces of the female die to obtain deep-drawn material. The system also includes a clamping mechanism for clamping at least one outer portion of the blank immediately adjacent the female die to guide the at least one outer portion of the blank to travel into the article-defining cavity at an acute angle with respect to the vertical axis during forcing of the inner portion of the blank into the female die so that thickness of at least one side wall of the deep-drawn material is substantially the same as thickness of the blank of thermoplastic sandwich material and so that the deep-drawn material does not significantly stretch or tear as the inner portion of the blank is forced into the female die.
The system preferably further includes a stamping press for forcing the male die into the female die.
Preferably, the clamping mechanism is spring-loaded so that the clamping mechanism exerts a clamping force at the at least one outer portion of the blank. The clamping force increases as the inner portion of the blank is forced into the female die.
The clamping mechanism preferably includes a clamping assembly mounted to move with the male die which has at least one spring which compresses as the inner portion of the blank is forced into the female die.
Preferably, the clamping mechanism includes a clamping assembly and a counter clamping assembly for clamping a plurality of spaced outer portions of the blank immediately adjacent the female die. A substantially constant distance is maintained between the clamping assembly and the counter clamping assembly as the inner portion of the blank is forced into the female die.
Each of the assemblies preferably includes a plurality of elongated clamping surfaces for clamping the plurality of spaced outer portions of the blank. Each elongated clamping surface is inclined at the acute angle with respect to the vertical axis.
The inner surfaces of the female die may define a plurality of corners which correspond to spaces between the plurality of spaced outer portions. The spaces are sized to permit thermoplastic sandwich material of the blank to move therein as the inner portion of the blank is forced into the female die.
Still further in carrying out the above objects and other objects of the present invention, a deep-drawn article formed by the steps of the method is provided.